64 TEKRESTRIAL MAGNETISM. 



limited to the sea and to its immediate vicinity) were far 

 from being contemporaneous. Towards the end of the 18th 

 century, the observations of the declination made with 

 better instruments, at fixed stations, by Cassini, Gilpin, and 

 Beaufoy (1781 to 1790), showed more decidedly a periodical 

 influence of hours and seasons, and gave a more animated 

 and general impulse to magnetic research. 



In the 19th century, of which little more than the half 

 has now elapsed, this branch of scientific inquiry has assumed 

 a peculiar character distinguishing it from all others. This 

 character consists in an almost simultaneous advance of all 

 parts of the study of terrestrial magnetism, in which physical 

 discoveries relating to the elicitation and distribution of mag- 

 netism, and the first and brilliant projection of a theory of ter- 

 restrial magnetism based on strict mathematical reasoning, by 

 Friedrich Gauss, have accompanied the unprecedented exten- 

 sion of numerical determinations of all the magnetic elements, 

 the declination, inclination, and intensity of the force. 



The means which have led to this result have been : 

 the improvement of instruments and methods of observation ; 

 scientific naval expeditions, on a scale and in number such 

 as no previous century had witnessed, carefully equipped at 

 the costs of the governments which sent them forth, and 

 favoured by a happy choice of commanders and observers ; 

 land journeys, penetrating far into the interior of continents ; 

 and lastly, the establishment of a considerable number of 

 fixed observatories, extending partially over both hemi- 

 spheres, in corresponding north and south latitudes, and in 

 almost antipodal longitudes. 



These magnetical and, at the same time, meteorological 

 observatories form a kind of net-work over the earth's 



